Thorium (chemical symbol Th) is a naturally-occurring radioactive metal found at very low levels in soil, rocks, and water. It has several different isotopes, both natural and man-made, all of which are radioactive. The most common form of thorium is thorium-232, found naturally.

The Basics

Who discovered thorium?

Thorium was discovered in 1828 by the Swedish chemist Jons Jakob Berzelius. After determining that it was a new element, Berzelius named his discovery after the Norse god of thunder and weather, Thor. Thorium was discovered to be radioactive independently in 1898 by Gerhard Carl Schmidt and by Marie Curie.

Where does thorium come from?

Almost all thorium is natural, but, thorium isotopes can be artificially produced. Thorium occurs at very low levels in virtually all rock, soil, and water, and therefore is found in plants and animals as well. Minerals such as monazite, thorite and thorianite are rich in thorium and may be mined for the metal. Generally, artificial isotopes come from decay of other man-made radionuclides, or absorption in nuclear reactions.

What are the properties of thorium?

Thorium is a soft, silvery white metal. Pure thorium will remain shiny for months in air, but if it contains impurities it tarnishes to black when exposed to air. When heated, thorium oxide glows bright white, a property that makes it useful in lantern mantles. It dissolves slowly in water. Thorium-232 has a half-life of 14 billion (14x109) years, and decays by alpha emission, with accompanying gamma radiation. Thorium-232 is the top of a long decay series that contains key radionuclides such as radium-228, its direct decay product, and radon-220. Two other isotopes of thorium, which can be significant in the environment, are thorium-230 and thorium-228. Both belong to other decay series. They also decay by alpha emission, with accompanying gamma radiation, and have half-lives of 75,400 years and 1.9 years, respectively.

What is thorium used for?

Thorium has coloring properties that has made it useful in ceramic glazes. But, it has been most widely used in lantern mantles for the brightness it imparts (though alternatives are replacing it), and in welding rods, which burn better with small amounts of added thorium. Thorium improves the properties of ophthalmic lenses, and is an alloying agent in certain metals used in the aerospace industry. More than 30 years ago, thorium oxides were used in hospitals to make certain kinds of diagnostic X-ray photographs. But, this practice has been discontinued.

Exposure to Thorium

How does thorium get into the environment?

Natural thorium is present in very small quantities in virtually all rock, soil, water, plants and animals. Where high concentrations occur in rock, thorium may be mined and refined, producing waste products such as mill tailings. If not properly controlled, wind and water can introduce the tailings into the wider environment. Commercial and federal facilities that have processed thorium may also have released thorium to the air, water, or soil. Man-made thorium isotopes are rare, and almost never enter the environment.

How does thorium change in the environment?

As thorium-232 undergoes radioactive decay, it emits an alpha particle, with accompanying gamma radiation, and forms radium-228. This process of releasing radiation and forming a new radionuclide continues until stable lead-208 is formed. The half-life of thorium-232 is about 14 billion years. Two other isotopes of thorium, which can be significant in the environment, are thorium-230 and thorium-228. Both decay by alpha emission, with accompanying gamma radiation, in 75,400 years and 1.9 years, respectively.

How are people exposed to thorium?

Since thorium is naturally present in the environment, people are exposed to tiny amounts in air, food and water. The amounts are usually very small and pose little health hazard. Thorium is also present in many consumer products such as ceramic glazes, lantern mantles, and welding rods.

People who live near a facility that mines or mills thorium, or manufactures products with thorium, may receive higher exposures. Also, people who work with thorium in various industries may receive higher exposures.

How does thorium get into the body?

People may inhale contaminated dust, or swallow thorium with food or water. Living near a thorium contaminated site, or working in an industry where thorium is used, increases your chance of exposure to thorium.

What does thorium do once it gets into the body?

If inhaled as dust, some thorium may remain in the lungs for long periods of time, depending on the chemical form. If ingested, thorium typically leaves the body through feces and urine within several days. The small amount of thorium left in the body will enter the bloodstream and be deposited in the bones where it may remain for many years. There is some evidence that the body may absorb thorium through the skin, but that would not likely be the primary means of entry.

Health Effects of Thorium

How can thorium affect people's health?

The principal concern from low to moderate level exposure to ionizing radiation is increased risk of cancer. Studies have shown that inhaling thorium dust causes an increased risk of developing lung cancer, and cancer of the pancreas. Bone cancer risk is also increased because thorium may be stored in bone.

Is there a medical test to determine exposure to thorium?

There are special tests that measure the level of thorium in the urine, feces, and also via exhaled air that can determine if a person has been exposed to thorium. These tests are useful only if taken within a week after exposure. You need special equipment to detect thorium not available in doctors offices or most hospitals. Some federal facilities and specialized laboratories have this capability.

Protecting People from Thorium

How do I know thorium if I'm near thorium?

What can I do to protect myself and my family from thorium?

Most people are not exposed to dangerous levels of thorium. However, people who live near thorium mining areas, or near certain government or industrial facilities may have increased exposure to thorium, especially if their water is from a private well. Analytical laboratories can test water for thorium content. Occasionally, household items may be found with thorium in them, such as some older ceramic wares in which uranium was used in the glaze, or gas lantern mantles. These generally do not pose serious health risks, but may nevertheless be retired from use as a prudent avoidance measure. A radiation counter is required to confirm if ceramics contain thorium.

What is EPA doing about thorium?

EPA protects people and the environment from thorium by establishing standards for the clean-up of contaminated sites, and by setting limits on the amount of thorium (and other radionuclides) that may be released to the air from specific sources, or found in public drinking water.

The standards for the clean-up of existing contaminated sites generally fall under the Comprehensive Environmental Response, Compensation, and Liability Act, commonly called Superfund. Clean ups must meet all requirements that are relevant or applicable, such as state regulations and regulations issued in connection with other environmental laws. When these types of regulations are not applicable, or not protective enough, EPA sets site-specific cleanup levels that limit the chance of developing cancer due to exposure to a site-related carcinogen (such as thorium) to between one in 10,000 and one in 1,000,000.

EPA issued special regulations for cleaning up uranium and thorium mill tailing sites under the "Uranium Mill Tailings Radiation Control Act" (federal regulations are found in 40CFR192, "Health and Environmental Protection Standards for Uranium and Thorium Mill Tailings"). These mills are found mostly in the western states of Colorado, Utah, Arizona and New Mexico.

Clean Air Act
EPA uses this authority to set limits on the emissions of hazardous air pollutants from specific sources. Hazardous air pollutants include both chemicals and radionuclides that are known or suspected to cause serious health problems. While no air emissions standards list thorium specifically, radionuclides are limited as a group.

RadNESHAPS
This site provides information on EPA's National Emission Standards for Hazardous Air Pollutants: Radionuclides.

Radionuclides in Drinking Water
This site provides information about radionuclides in drinking water and guidance to help states and water systems comply with the standard. EPA uses its Safe Drinking Water Act authority to establish maximum contaminant levels (MCLs) for alpha emitters such as thorium in public drinking water. The MCL for alpha emitters is 15 picoCuries per liter of water.